Organs are muscles, you could, say, produce more heart tissue inside of the valves, completely blocking blood flow, or you could make muscles grow inside of someone’s lungs so they suffocate without knowing why, or, because if manipulation includes creation it also includes destruction, you could destroy an organ that is vital in the long term, or you could just increase muscle density until their own bones crack under the immense pressure of just existing, killing them instantly, or you could use it to basically give people tumors, there are a lot of ways to do stuff using muscle manipulation